Abstract:
The Dairy Gas Emissions Model (DairyGEM) is a software tool for estimating ammonia, hydrogen sulfide, and greenhouse gas (GHG) emissions of dairy production systems as influenced by climate and farm management. A dairy production system generally represents the processes used on a given farm, but the full system may extend beyond the farm boundaries. A production system is defined to include ... emissions during the production of all feeds whether produced on the given farm or elsewhere. It also includes GHG emissions that occur during the production of resources used on the farm such as machinery, fuel, electricity, and fertilizer. Manure is assumed to be applied to cropland producing feed, but any portion of the manure produced can be exported to other uses external to the system.

DairyGEM uses process level simulation to predict ammonia and hydrogen sulfide emissions from manure in the housing facility, during long term storage, following field application and during grazing. Process-based relationships and emission factors are used to predict the primary GHG emissions from the production system. Primary sources include the net emission of carbon dioxide plus all emissions of methane and nitrous oxide. All emissions are predicted through a daily simulation of feed use and manure handling. Daily emission values of each gas are summed to obtain annual values.

Ammonia emissions occur from the barn floor, during manure storage, following field application, and during grazing. Barn floor emissions are determined separately for cow and replacement heifer facilities. For each facility, hourly emission rates are a function of the type of housing facility, the nitrogen level in excreted manure, temperature, air velocity, and other factors. When long term manure storage is used, ammonia emissions continue from the storage facility as a function of manure nitrogen and solids content, storage design, temperature, and wind velocity. Following field application of manure, ammonia is rapidly emitted unless it is incorporated by a tillage operation or directly injected into the soil. For grazing animals, ammonia is emitted from urine patches where the emission rate is again a function of temperature and wind velocity.

Hydrogen sulfide emissions are predicted using a process-based model similar to that used for ammonia. Since hydrogen sulfide is created under anaerobic conditions, most of this emission occurs during anaerobic storage of manure. The barn floor or drylot may also be an important emitter with minor emissions following field application and during grazing. Emissions from the barn floor are related to the sulfide content of the manure, manure pH, air temperature, and air velocity. These same factors influence emissions during long-term storage where the anaerobic conditions are conducive to sulfide production. When stored manure is broadcast on fields, any sulfide remaining in the manure is quickly lost and further formation ceases under these aerobic conditions. Very small amounts of hydrogen sulfide are produced and released from feces deposits on pasture as influenced by temperature.

Carbon dioxide emissions include the net annual flux in feed production and daily values from animal respiration and microbial respiration in manure on the barn floor and during storage. The annual flux in feed production is that assimilated in the feed minus that in manure applied to cropland. Emission of carbon dioxide through animal respiration is a function of animal mass and daily feed intake, and that from the floor is a function of ambient or barn temperature and the floor surface area covered by manure. Emission from a manure storage is predicted as a function of the volume of manure in the storage using an emission factor. Finally, carbon dioxide emission from fuel combustion in farm engines is proportional to the amount of fuel used in the production and feeding of feeds and the handling of manure.

Methane emissions include those from enteric fermentation, the barn floor, manure storage, and feces deposited in pasture. Emission from enteric fermentation is a function of the metabolizable energy intake and the diet starch and fiber contents for the animal groups making up the herd. Daily emissions from the manure storage are a function of the amount of manure in the storage and the volatile solids content and temperature of the manure. Emissions following field application of manure are related to the volatile fatty acid content of the manure and the amount of manure applied. Emissions during grazing are proportional to the amount of feces deposited on the pasture; that emitted in the barn is a function of the amount of manure deposited in the barn, barn temperature, and the floor area covered by the manure.

Nitrous oxide emissions are that emitted from crop and pasture land during the production of feeds with minor emissions from the manure storage and barn floor. An emission factor approach is used to estimate annual emissions in feed production where the emission is 1% of the fertilizer and manure N applied to cropland and 2% of that applied to pastureland. Emission from the crust on a slurry manure storage is a function of the exposed surface area.

Total greenhouse gas emission is determined as the sum of the net emissions of the three greenhouse gases where methane and nitrous oxide are converted to carbon dioxide equivalent units (CO2e). The carbon footprint of milk production is this net of all greenhouse gas emissions divided by the milk produced. The net emission is determined through a partial life cycle assessment (LCA) of the production system including both primary and secondary sources. Primary emissions are those emitted from the farm or production system during the production process. Secondary emissions are those that occur during the manufacture or production of resources used in the production system. These resources include machinery, fuel, electricity, fertilizer, pesticides, and plastic.